1. Field of the Invention
This invention relates in general to subsea wells, and in particular to a system for reconnecting a riser from a floating vessel to a subsea well for workover operations.
2. Description of the Prior Art
In deep water offshore oil and gas wells, the Christmas tree of the well will often be located on the subsea floor. At times, a workover operation must be performed on the subsea well. When this is required, a floating vessel is positioned over the well. A string of riser pipe is lowered down into engagement with a mandrel on the subsea tree. Once in engagement, operations can be performed on the well.
If the system is a guidelineless system, there will be no guidelines extending upward from the subsea well structure to the surface. Generally, in a guidelineless system, a large upward facing funnel is mounted permanently on the subsea tree. The funnel, with the aid of television cameras, assists in guiding the lower end of the riser onto the mandrel of the subsea well. The funnel can be quite large, up to twelve feet in diameter. A funnel of this type is expensive to construct and is only used when a workover operation is performed.
Mounting a downward facing funnel on the riser would avoid the need for a permanent upward facing funnel on each well. However, a funnel rigidly mounted to the lower end of the riser would require an extra high mandrel extending above the control mechanisms on the tree, so as to insure that the funnel did not strike any of various control mechanisms on the side of the tree. Hydraulic connections must also be made up when the riser lands on a mandrel to connect the control of the tree to the floating platform. Orienting the funnel onto the mandrel of the Christmas tree without damage to the hydraulic manifold or valve block would be a problem.
There have been proposals to make the funnel retractable. The funnel would be located on the lower end of the riser, but would be vertically movable relative to the lower end of the riser by means of hydraulic rams. The funnel is lowered in an extended position. Once proper orientation has been made, the funnel would be retracted. During retraction, the riser and mandrel connector lower down into engagement with the mandrel. While these proposals have merit, improvements are desirable.